Process of making chromic acid



Patented Dec. 10, 1929 UNITED STATES PATENT OFFICE- v MARVIN J. UDY, OF NIAGARA FALLS, NEW YORK PROCESS OF MAKIN 'G CHROMIC .ACIIJ No Drawing.

a process ofmaking that surrounding the anode and to keep the chromic acid formed away from the vicinity of the cathode. According to this prior process the electrolyte in the cathode compartment consists initially of sulfuric acid or a soluble sulfate, anode compartment consists initially of a suspension of chromic hydroxide in water slightly acidified with sulfuric acid or containing a soluble sulfate; As this process has been conducted, a product containing not over 10% of CrO has been obtained.

Ihave found that little or no reduction of chromic acid takes place at the cathode if an electrolyte containing chromic acid is used and thecathode is immersed directly therein. I have also found that when chromic hydroxide is added to a chromic acid solution it dissolves quite readily therein. Accordingly, by using a chromic acid electrolyte, the use of a W diaphragm separating the cell into two compartments may be dispensed with. It is also possible to produce either intermittently or as a continuous process a relatively concentrated chromic acid solution substantially free from impurities.

According to the preferred form of my invention, chromic hydroxide, Cr(OH) preferably freshly prepared and in a moist condition, is fed to an electrolytic bath consisting of an aqueous solution of chromic acid. The rate of feed should be as even as possible and such as to maintain the voltage .5 to 1.0 volt higher than that characteristic of the operation when no chromic hydroxide is present. The chromic 59 tion, presumably as the result of a chemical and the electrolyte in the hydroxide dissolves in the chromic acid solub Application filed'November 5, 1925. Serial No. 67,152.

reaction inwhich chromium chromate is formed according to the following equation: 2011011) H2cro.= c'r 0,. cr0 413 0 The oxidation of the chromate at the anode may be represented by the following equat1on:

Cr O .Cr0 snzo 30=3H CtO The current density may be varied from a fraction of an ampere to 300 amperes per sq. 60 ft., but the best results are obtained when operating Within the range 100 to 200 amperes per sq. ft. It is preferred to maintain the temperature of the bath at from 95 C. to

100 C. I prefer to use a lead anode and a cathode of aluminum, iron or other suitable metal. The anode is preferably of the hollow type to permit of the circulation of steam or hot Water through it to maintain the temperature of the bath within the limits specitied.

The concentration of electrolyte preferably should be maintained between 15 and 30%. When the operation is carried out as a continuous process, chromic acid should be withdrawn at such rate as to maintain the concentration of the electrolyte within the limits specified. According to this inventiona product consisting of an aqueous solution having a CrO concentration of from 20 to 30% is regularly obtained, while higher concentrations may be obtained if desired.

The product consists of a substantially pure chromic acid solution which may be used without recrystallization for many purposes requiring pure chromic acid. A high current efliciency is obtained.

Best results are obtained by the use .of chromic acid electrolyte free from mineral acids, although the process may be carried out with sulfuric acid present in small chromic acid in the 'amounts not exceeding about one-half of one per cent.

I claim:

1. Process of making chromic acid which comprises passin current between an--insoluble anode an a cathode in contact with an electrolyte containing trivalent chronium etween approximately 15% and 30% Grog, the composition of the electrolyte throughout the space between the electrodes being substantially uniform.

- 2. Process of making chromic acid which comprises .passing current between an in: soluble anode and a cathode in'contact with an electrolyte containin between approximately and 30% grO addingtrivalent chromium to theelectrolyte, and converting the trivalent chromium into chromic acid by anodic oxidation. 3. Process of making chromic acid which comprises passing current between an insoluble anode and a cathode in contact with an electrolyte containing between approximately 15% and 30% CrO adding chromic hydroxide to the electrolyte, and converting the chromic hydroxide into chromic acid by anodic oxidation.

4. Process of making chromic acid which comprises passing current between an insoluble anode and a cathode in contact with an electrolyte consisting principally of an aqueous solution of hexavalent chromium, maintaining the composition of the electrolyte throughout the space between the electrodes substantially uniform, adding trivalent chromium to the electrolyte and converting the trivalent chromium into chromic acid by anodic oxidation.

a0 5. Process of making chromic acid which comprises passing current between an insoluble anode and a cathode in contact with an electrolyte consisting principally of an aqueous solution of chromic acid, maintaining the composition of the electrolyte throughout the space between the electrodes substantially uniform, adding trivalent chromium to the electrolyte and converting the trivalent chromium into chromic acid by anodic oxidation.

6. Process of making chromic acid which comprises passing current between an in-' soluble anode and a cathode in contact with an electrolyte consisting principally of an aqueous solution of chromic acid and trivalent chromium, maintaining the composition of the electrolyte throughout the space between the electrodes substantially uniform, converting the trivalent chromium into so chromic acid by anodic oxidation, continuously removing chromic acid, and adding water and trivalent chromium so as to maintain the conce tration of the electrolyte within predetermined limits.

;n testimony whereof, I afiix my signature.

' MARVIN J. UDY. 

